1. Field of the Invention
The present invention relates to a ceramic dynamic-pressure bearing, a motor having a bearing, a hard disk drive, and a polygon scanner.
2. Description of the Related Art
Conventionally, a ball bearing has often been used as a bearing for the shaft of a motor serving as a drive unit of electric equipment. High-speed rotation of a motor has been rapidly implemented in precision equipment, such as peripheral equipment of a computer. In this connection, in order to obtain excellent bearing performance with low rotation-speed fluctuation and reduced noise and vibration, or in order to elongate bearing service life, a dynamic-pressure bearing, which uses fluid, such as air, as a medium, has been employed. The dynamic-pressure bearing operates in the following manner: when, for example, a spindle and a bearing member disposed so as to surround the spindle undergo relative rotation about an axis, the axis of rotation is supported by the action of fluid dynamic-pressure generated in the gap formed between the outer circumferential surface of the spindle and the inner circumferential surface of the bearing member. Also, another bearing is configured such that the thrust face of a spindle or that of a bearing member is supported by action of dynamic pressure.
When a dynamic-pressure bearing is in a high-speed rotation state, in which generated dynamic-pressure is sufficiently high, two members which face each other with a dynamic-pressure gap present therebetween do not come into contact with each other. However, at the time of starting or stopping, when rotational speed is low, sufficiently high dynamic pressure is not generated; thus, the two members come into contact with each other. Component members of such a dynamic-pressure bearing have generally been formed of a metal, such as stainless steel, and in some cases have been further coated with resin or a like material. However, the two metallic members are subject to a problem of wear or seize-up caused by mutual contact thereof at the time of starting or stopping. In order to prevent this problem, coating a metallic member with a lubricating layer, such as a resin layer, at a portion facing the dynamic-pressure gap has been attempted, resulting in failure to yield a sufficient effect. In order to attain sufficient endurance against wear and seize-up, either or both of the two members, such as either or both of the spindle and the bearing member described above, which face each other with a dynamic-pressure gap present therebetween have been formed of a ceramic, such as alumina.
3. Problems to be Solved by the Invention
However, conventionally, when a dynamic-pressure component is formed of alumina ceramic, material design cannot be said to have sufficiently considered wear or a like problem associated with starting, stopping, or a like operation mode. Also, even when a component of a dynamic-pressure bearing is formed of ceramic, a problem may arise such that vibration occurs during rotation of a spindle, thereby hindering smooth rotation of the spindle. In a dynamic-pressure bearing configured so as to support a thrust face by the action of dynamic pressure, such as a dynamic-pressure bearing configured such that a thrust face of a rotation body faces a disk-like thrust plate, when the rotation body and the thrust plate come into contact with each other at the time of starting or stopping, wear or linking (a phenomenon in which two members come into close contact due to vacuum created in the clearance therebetween) may arise.
It is therefore an object of the present invention to provide a ceramic dynamic-pressure bearing which is not prone to wear or a like problem associated with starting, stopping, or a like operation mode and which can realize smooth rotation.
The above object of the invention has been achieved by providing a ceramic dynamic-pressure bearing having a dynamic-pressure gap formed between a first member and a second member. The first and second members undergo relative rotation about a predetermined axis of rotation to generate fluid dynamic-pressure in the dynamic-pressure gap. At least a portion of one or both of the first member and the second member including a dynamic-pressure gap definition surface formed of alumina ceramic and comprising a polished surface facing the dynamic-pressure gap. Furthermore, the alumina ceramic, which forms the dynamic-pressure gap definition surface finished by polishing, has an apparent density of 3.5-3.9 g/cm3.
The present invention uses alumina ceramic. Alumina is relatively inexpensive and exhibits high strength and excellent chemical stability. In the present invention, the density of the alumina ceramic is adjusted to a relatively high value of 3.5-3.9 g/cm3, thereby improving the absolute value of strength and wear resistance of the alumina ceramic. The material is used to form the dynamic-pressure gap definition surface, thereby effectively preventing occurrence of wear and seize-up of the dynamic-pressure gap definition surface at the time of starting and stopping when the two members are prone to come into contact with each other.
An ideally densified alumina ceramic has a density of up to 4.0 g/cm3. However, the present invention particularly employs a slightly lower density of 3.9 g/cm3 as the upper limit of apparent density, for the following reason. When alumina ceramic is used as material for a dynamic-pressure bearing, the surface state of the dynamic-pressure gap definition surface of the ceramic component serving as a spindle or a bearing is important. That is, in general, fine pores are present on the surface of the ceramic component that has been subjected to polishing, and the size of such pores is considered to exert considerable influence on the state of rotation of the dynamic-pressure bearing.
Studies carried out by the present inventors have revealed that an extremely smooth dynamic-pressure gap definition surface fails to generate sufficient fluid dynamic-pressure in a dynamic-pressure gap. Insufficient dynamic pressure fails to stably support the axis of rotation, resulting in difficulty in establishing a favorable state of rotation of a dynamic-pressure bearing. Accordingly, formation of surface pores of a certain dimensional range on the dynamic-pressure gap definition surface is effective for maintaining high fluid dynamic-pressure that is generated stably at a high level.
Specifically, when pores of large size are present on the dynamic-pressure gap definition surface of the ceramic component, turbulence is generated in the fluid layer present between the spindle and the bearing upon rotation of, for example, the spindle, with the result that vibration of the spindle occurs. By contrast, when pores of small size are present on the dynamic-pressure gap definition surface of the ceramic component, adhesion easily occurs between the dynamic-pressure gap definition surfaces of the spindle and the bearing, with the result that, for example, an attempt to forcibly induce rotation in a high-friction state associated with adhesion is likely to cause occurrence of wear (hereinafter referred to as xe2x80x9cadhesion wearxe2x80x9d) or a like problem. When one of two members between which a dynamic-pressure gap is formed is formed of metal; for example, when the spindle is formed of a metal, seize-up may occur. Also, surface pores of excessively small size hardly contribute to generation of dynamic pressure.
The above-mentioned pores are formed on the dynamic-pressure gap definition surface mainly as a result of dropping off of grains in the course of polishing. Thus, the size (diameter) or distribution of crystal grains of alumina ceramic on the dynamic-pressure gap definition surface plays a very important role in forming surface pores in a favorable state against occurrence of the above-described problems. For example, when alumina ceramic is to be sintered for complete densification, sintering must be performed at high temperature, with the result that the growth of crystal grains becomes unavoidable. In such a case, dropping off of grains becomes unlikely in the course of polishing, resulting in an insufficient amount of formed surface pores, and consequently wear adhesion or a like problem is likely to occur at the time of starting or stopping. Also, dropping off of a grown crystal grain results in formation of an excessively large surface pore, resulting in difficulty in maintaining stable rotation. However, when the upper limit of apparent density of the alumina ceramic is set to about 3.9 g/cm3, the sintering temperature does not need to be increased much, thereby improving the state of formation of surface pores by restraining the growth of crystal grains. As a result, employing an alumina ceramic of relatively high density yields a combined effect of enhancing the absolute value of strength and wear resistance and effectively solves the problem of wear on the dynamic-pressure gap definition surface or the problem of linking. An apparent density of less than 3.5 g/cm3 impairs strength and wear resistance of alumina ceramic, with the result that the dynamic-pressure gap definition surface becomes likely to suffer wear at the time of starting or stopping. More preferably, the apparent density of the alumina ceramic is adjusted to 3.6-3.9 g/cm3.
The apparent density of the alumina ceramic is not only influenced by the condition of densification, but, to some extent, is also influenced by the kind and content of a sintering aid added thereto. Relative density (i.e., a value obtained by dividing apparent density by true density estimated from the compositional ratios of alumina and a sintering aid) can be used as an index for describing the relationship between a densification level and the degree of growth of crystal grains of ceramic. In the present invention, the relative density of the alumina ceramic is 90-98%, preferably 94-98%.
An alumina ceramic adjusted to the above-described density range can assume a high bending strength of 280-550 MPa. The dynamic-pressure gap definition surface of the alumina ceramic can attain a high Rockwell hardness of 92-98 measured at a load of 15N. Enhancement of the strength of alumina ceramic to such a high level effectively prevents or restrains wear on the dynamic-pressure gap definition surface at the time of starting or stopping. Notably, bending strength herein refers to 3-point bending strength measured at room temperature according to the method specified in JIS R1601 (1981). Rockwell hardness measured at a load of 15 N refers to hardness measured at room temperature according to the method specified in JIS Z2245 (1992).
The ceramic which is used to form the dynamic-pressure gap definition surface of a member can be adjusted such that the crystal grains assume an average grain size of 1-7 xcexcm. As a result, the size and amount of surface pores which are advantageous can be realized in terms of stable maintenance of generated fluid dynamic-pressure at a high level and effective restraint of a problem, such as adhesion wear, seize-up, linking, at the time of starting or stopping a dynamic-pressure bearing. Adjusting the ceramic crystal grains to an average grain size of 1-7 xcexcm, which is a rather small value for alumina ceramic, enhances the mechanical strength of alumina ceramic, thereby enhancing wear resistance.
Herein, the size of a surface pore (or a crystal grain) is defined in the following manner. As shown in FIG. 6, various parallel lines circumscribe a surface pore (or crystal grain) which is observed on the microstructure of the dynamic-pressure gap definition surface by means of SEM, an optical microscope, or like equipment. The size of the surface pore is represented by an average value of the minimum distance dmin between such parallel lines and the maximum distance dmax between such parallel lines (i.e., d =(dmin +dmax)/2).
When ceramic crystal grains assume an average grain size of less than 1 xcexcm, the average size of surface pores to be formed becomes too small, and as a result the dynamic-pressure gap definition surface is prone to adhesion wear, seize-up, or linking when the bearing starts or stops rotating. Also, since fluid dynamic-pressure to be generated in the dynamic-pressure gap tends to become insufficient, rotational runout becomes likely to occur. By contrast, when the ceramic crystal grains assume an average grain size in excess of 7 xcexcm, the average size of surface pores to be formed becomes too large, with the result that excessive turbulence is generated in the dynamic-pressure gap, and thus the axis of rotation is likely to vibrate. More preferably, the ceramic crystal grains assume an average grain size of 2-5 xcexcm.
In order to realize the above-mentioned advantageous size and amount of surface pores formed on the dynamic-pressure gap definition surface, ceramic crystal grains having a grain size of 2-5 xcexcm preferably occupy an area percentage of not less than 40% (including 100%). When an area percentage occupied by ceramic crystal grains falling within the above-mentioned dimensional range is less than 40%; for example, when grains having a grain size in excess of the upper limit of the above-mentioned dimensional range increase, dropping off of grains becomes unlikely to occur, with the result that an area percentage occupied by surface pores contributing effectively to generation of dynamic pressure may become insufficient. By contrast, when grains having a grain size less than the lower limit of the above-mentioned dimensional range increase, the average size of surface pores to be formed tends to decrease. Either case may be disadvantageous in terms of generation of sufficient dynamic pressure. In view of prevention of occurrence of vibration, preferably the maximum size of surface pores present on the dynamic-pressure gap definition surface is not greater than 100 xcexcm; i.e., surface pores having a size in excess of 100 xcexcm should not be present.
Alumina ceramic can be made by sintering a mixture of an alumina powder and an appropriate sintering aid powder (e.g., a powder of an oxide of Mg, Ca, Si, or Na). As shown in FIG. 11, the alumina ceramic assumes a microstructure such that crystal grains, which contain a predominant amount of alumina and form a main phase, are joined together by a grain boundary phase derived from the sintering aid. Dropping off of crystal grains in the course of polishing is considered to occur mainly through fracture of the grain boundary phase. As a result of crystal grains dropping off, spaces which have been occupied by the crystal grains open on the dynamic-pressure gap definition surface to thereby become pores. Conceivably, crystal grains are likely to drop off in a portion of the grain boundary phase where the bonding force is relatively weak, such as a portion of the grain boundary phase where the thickness of the grain boundary phase is decreased, a portion of the grain boundary phase where the grain boundary phase is not present due to presence of an internal cavity or the like, or a portion of the grain boundary phase where the strength of the grain boundary phase is lacking due to, for example, the presence of a crack derived from component segregation, thermal stress, or a like cause. Notably, in the present invention, unless otherwise specified, the term xe2x80x9cpredominantxe2x80x9d used in relation to content means that the substance in question is contained in an amount of not less than 50% by weight (the terms xe2x80x9cpredominantlyxe2x80x9d and xe2x80x9cmainlyxe2x80x9d have the same meaning).
For example, when a single crystal grain drops off,.a pore whose shape and size correspond to those of the crystal grain is formed as represented by pore V1 in FIG. 12(a) (in the figure, white grains represent remaining grains, whereas black grains represent grains which have dropped off). When a plurality of crystal grains drop off, a pore as represented by V2 is formed. As shown in FIG. 12(b), the microstructure of ceramic is usually such that crystal grains of various sizes are mixedly present. Thus, when a large crystal grain is surrounded by a plurality of small crystal grains, dropping off of a series of the smaller crystal grains may cause dropping off of the central large crystal grain. In these cases, a pore to be formed naturally becomes greater in size than the individual crystal grains which have dropped off.
When the microstructure of alumina ceramic is isometric; i.e., the shape anisotropy of individual crystal grains is low, and a portion of the grain boundary phase where a bonding force is weakened spreads to a certain extent, the form of dropping off as represented by V2 tends to occur at higher frequency upon application of a polishing force on a plurality of crystal grains from a grinding wheel or abrasive grains. In this case, the average size of surface pores to be formed becomes greater than the average grain size of crystal grains adjusted to a grain size of 1-7 xcexcm. Surface pores are formed on the dynamic-pressure gap definition surface in an isotropically scattered fashion, rather than in a fashion scattered in the polishing direction. As a result of surface pores assuming an average size greater than the average grain size of crystal grains, dynamic pressure to be generated can be increased further, thereby realizing stabler rotation of the bearing.
In production of alumina ceramic assuming the above-described microstructure peculiar to the present invention, preferably, an alumina powder used as a material has an average particle size of 1-5 xcexcm. When an alumina powder whose average particle size falls outside the range is used, the sintered body thereby obtained may fail to have an average grain size falling within the previously described preferable range. The average particle size of a powder can be measured by use of a laser diffraction granulometer.
Preferably, the firing temperature falls within a range of 140xc2x0 C. to 1700xc2x0 C. When the firing temperature is lower than 1400xc2x0 C., a sintered body encounters difficulty in undergoing densification, resulting in a failure to assume sufficient strength or wear resistance. By contrast, when the firing temperature is in excess of 1700xc2x0 C., excessive grain growth occurs, and consequently crystal grains of a sintered body thereby obtained may fail to assume an average grain size that falls within the previously mentioned preferable range. Also, a sintered body is prone to suffer deformation or a like problem, with the result that dimensional accuracy may be impaired.
Preferably, the amount of a sintering aid component as reduced to an oxide thereof is 0.5-10% by weight. When the sintering aid component content is less than 0.5% by weight, a sintered body encounters difficulty in undergoing densification, possibly resulting in a failure to assume sufficient strength or wear resistance. When the sintering aid component content is at such a low level, a liquid phase is hardly generated in the course of sintering, with the result that the growth of crystal grains is restrained. In this case, use of a material powder having the above-mentioned average particle size may raise a problem in that the sintered body encounters difficulty in assuming an average crystal grain size that falls within the previously mentioned range peculiar to the present invention, unless crystal grains grow to a certain extent. By contrast, when the sintering aid component content is in excess of 10% by weight, the strength and wear resistance of a sintered body may be impaired. Preferably, the Al component content of alumina ceramic as reduced to Al2O3 is 90-99.9% by mass for enhancing strength and toughness, as well as the wear resistance of the dynamic-pressure gap definition surface which is formed of the alumina ceramic.
Preferably, surface pores present on the dynamic-pressure gap definition surface formed of a ceramic assume an average size of 2-20 xcexcm. Through active formation of surface pores having an average size of 2-20 xcexcm, the generated fluid dynamic-pressure can be maintained stably at a high level. Further, in the case of a dynamic-pressure bearing having a thrust dynamic-pressure gap, which will be described below, the incidence of linking can be prevented.
When the average size of surface pores is in excess of 20 xcexcm, excessive turbulence is generated in the dynamic-pressure gap, with the result that the axis of rotation is likely to undergo vibration. By contrast, when the average size of surface pores is less than 2 xcexcm, the dynamic-pressure gap definition surface is prone to suffer adhesion wear, seize-up, or linking when the bearing starts or stops rotating. Also, since fluid dynamic-pressure to be generated in the dynamic-pressure gap tends to become insufficient, rotational runout becomes likely to occur. More preferably, the average size of surface pores is 5-15 xcexcm.
Preferably, the dynamic-pressure gap definition surface is coated with a hard carbon film formed mainly of amorphous carbon and having a thickness smaller than the average size of surface pores. The film prevents potential occurrence of wear and adhesion even when the dynamic-pressure gap definition surfaces come into contact with each other in a state of low-speed rotation, which arises at the time of starting or stopping and tends to involve the occurrence of insufficient dynamic pressure. The xe2x80x9chard carbon film formed mainly of amorphous carbonxe2x80x9d refers to a film whose skeleton texture serving as its main body is amorphous and whose Vickers hardness is not less than 1500 kg/mm2. The hardness of the film can be measured by use of, for example, a dynamic, ultra-low hardness tester (e.g., NHT, product of CSEM Instruments in Switzerland). The average thickness of the film is rendered smaller than the average size of surface pores in order to prevent excessive blockage of surface pores, which are actively formed for enhancement of a dynamic-pressure generation effect.
The hard carbon film can be formed by the method described in Japanese Patent Publication (kokoku) No. H06-60404. In this case, preferably, in order to effectively deposit a hard carbon film so as not to block surface pores, which contribute to generation of dynamic pressure, the vapor of a material to be deposited is impinged obliquely on the surface of a member for film deposition.
Surface pores having a size of not greater than 2 xcexcm cannot contribute much to the generation of dynamic pressure. By contrast, when surface pores having a size in excess of 20 xcexcm are present in excessive amount, vibration or a like problem is likely to occur. That is, in order to effectively generate dynamic pressure and to realize stable rotation, the size of surface pores is preferably 2-20 xcexcm. In order to effectively restrain subjecting the dynamic-pressure gap definition surface to seize-up or linking at the time of starting or stopping rotation and to increase fluid dynamic-pressure to be generated in the dynamic-pressure gap, surface pores whose size falls within the above-mentioned range preferably occupy an area percentage of not less than 10%, more preferably not less than 15%, on the dynamic-pressure gap definition surface. In view of effective restraint of occurrence of vibration or a like problem, the area percentage is preferably not greater than 60%, more preferably not greater than 40%.
More preferably, in order to effectively contribute to generation of dynamic pressure and to realize stable rotation, the surface pores assume a size of 5-15 xcexcm, and surface pores whose size falls within the dimensional range occupy an area percentage of 15-30% on the dynamic-pressure gap definition surface.
Herein, the term xe2x80x9carea percentage of surface poresxe2x80x9d is a value obtained by dividing the total area of pores observed on the dynamic-pressure gap definition surface by the area of the dynamic-pressure gap definition surface. When known dynamic-pressure grooves are formed on the dynamic-pressure gap definition surface, the area of an effective dynamic-pressure gap definition region after exclusion of the dynamic-pressure grooves from the dynamic-pressure gap definition surface is used for calculating the area percentage of surface pores. The area percentage is measured by the steps of observing the effective dynamic-pressure gap definition region by use of magnifying observation means, such as an optical microscope; determining a square measurement region measuring 300 xcexcmxc3x97300 xcexcm within the field of observation; and dividing the total area of surface pores observed within the measurement region by the area of the measurement region. Preferably, in order to improve measurement accuracy, five or more measurement regions are arbitrarily determined within a single effective dynamic-pressure gap definition region, and the area percentage of surface pores is obtained by averaging area percentage values of surface pores of the measurement regions.
Preferably, the dynamic-pressure gap definition surface is free, to the greatest possible extent, from surface pores having a size in excess of 20 xcexcm, since such surface pores are likely to cause occurrence of vibration or a like problem. Specifically, surface pores having a size in excess of 20 xcexcm occupy an area percentage of not greater than 10%, preferably not greater than 5%, on the dynamic-pressure gap definition surface. In view of prevention of vibration, preferably, the maximum size of surface pores present on the dynamic-pressure gap definition surface is not greater than 100 xcexcm; i.e., surface pores having a size in excess of 100 xcexcm are not present.
The first member and the second member, which define a dynamic-pressure gap therebetween, can be formed entirely of alumina ceramic (hereinafter also referred to as xe2x80x9cceramicxe2x80x9d). Preferably, the ceramic, which is used to form the members, is a densely sintered body whose microstructure is such that few pores are formed internally, whereas pores are formed in a relatively large amount on the dynamic-pressure gap definition surface, in view of increase of dynamic pressure to be generated, effective prevention of adhesion wear, seize-up, or linking, and enhancement of strength and wear resistance. Specifically, preferably, pores having a size of 2-20 xcexcm present in the ceramic sintered body are localized mainly on the dynamic-pressure gap definition surface in the form of surface pores. Such a microstructure is efficiently attained by the previously described method, in which ceramic crystal grains are caused to drop off to thereby form surface pores in the course of finishing the dynamic-pressure gap definition surface.
The dynamic-pressure gap definition surface can be a radial dynamic-pressure gap definition surface located radially distant from the axis of rotation of the bearing. Specifically, the first member is formed into a spindle and is inserted into a reception hole formed in the second member; and the inner surface of the reception hole and the outer circumferential surface of the first member to be received inside the inner surface serve as radial dynamic-pressure gap definition surfaces, which define a radial dynamic-pressure gap therebetween.
For example, in a dynamic-pressure bearing having a structure shown in FIG. 1, the radial direction is a direction perpendicular to the axis of rotation (extending vertically in FIG. 1) of the spindle. For example, in FIG. 1, the outer circumferential surface of a spindlexe2x80x94which serves as the first member in a fixed conditionxe2x80x94and the inner circumferential surface of a bearing member which serves as the second member assuming the form of a cylindrical rotation bodyxe2x80x94serve as the radial dynamic-pressure gap definition surfaces. As will be described below, in the case of a bearing elongated along the axis of rotation, whether or not radial dynamic-pressure is sufficiently generated determines whether or not the axis of rotation is supported stably. Application of the present invention to a bearing allows generation of sufficient dynamic pressure in the radial dynamic-pressure gap and effectively prevents or restrains adhesion wear, seize-up, or a like problem at the time of starting and stopping.
The dynamic-pressure gap definition surface can be a thrust dynamic-pressure gap definition surface formed at a certain location in the thrust direction relative to the axis of the rotation body. Specifically, the first member is disposed to face at least one end face of the second member with respect to the axis of rotation; and the end face of the second member and a face of the first member facing the end face serve as the thrust dynamic-pressure gap definition surfaces, which define a thrust dynamic-pressure gap therebetween.
For example, in the dynamic-pressure bearing having a structure shown in FIG. 1, the thrust direction is the axial direction of the spindle; i.e., a direction along which the axis of rotation extends (the vertical direction in FIG. 1). For example, in FIG. 1, an end face of the bearing memberxe2x80x94which serves as the second member assuming the form of a cylindrical rotation bodyxe2x80x94and a face of a thrust platexe2x80x94which serves as the first member facing the end face of the bearing member with respect to the axis of rotationxe2x80x94serve as the thrust dynamic-pressure gap definition surfaces. The thrust dynamic-pressure gap definition surfaces may be slightly inclined from a plane perpendicular to the axis of rotation. As will be described below, in the case of a bearing which is of short length along the axis of rotation, whether or not radial dynamic-pressure is sufficiently generated determines whether or not the axis of rotation is stably supported. Application of the present invention to a bearing allows generation of sufficient dynamic pressure in the thrust dynamic-pressure gap and effectively prevents or restrains adhesion wear, seize-up, or linking at the time of starting and stopping.
As shown in FIG. 1, a single bearing can have both a radial dynamic-pressure gap and a thrust dynamic-pressure gap. In this case, the first member (or the second member) as viewed from the standpoint of the radial dynamic-pressure gap and the first member (or the second member) as viewed from the standpoint of the thrust dynamic-pressure gap may be the same member or mutually different members depending on the form of the dynamic-pressure gaps. For example, in the case of FIG. 1, the second member is the bearing member as viewed from the standpoint of either dynamic-pressure gap; and the inner circumferential surface of the bearing member serves as the radial dynamic-pressure gap definition surface, whereas the opposite end faces of the bearing member serve as the thrust dynamic-pressure gap definition surfaces. As for the first member, the spindle is the first member as viewed from the standpoint of the radial dynamic-pressure gap, whereas a pair of thrust plates facing the corresponding opposite end faces of the bearing member is the first member as viewed from the standpoint of the thrust dynamic-pressure gap. The spindle is a nonrotating fixed shaft. Notably, as shown in FIG. 10, a bearing 251 is configured such that a spindle 212 is a rotating member, whereas a cylindrical bearing member 221 is a fixed member.
The dynamic-pressure bearing of the present invention can be configured such that the axial length thereof is longer than the outside diameter of the thrust dynamic-pressure gap definition surface, or a thrust dynamic-pressure gap is not formed and such that the inclination of the rotation body during rotation is restricted by dynamic pressure generated in the radial dynamic-pressure gap. This defines, for example, a dynamic-pressure bearing having a long axial length as shown in FIG. 7. When a bearing member 35 serving as a rotation body inclines, the inclination is corrected by the action of pressure generated in a radial dynamic-pressure gap 37. By contrast, the dynamic-pressure bearing can also be configured such that the axial length thereof is shorter than the outside diameter of the thrust dynamic-pressure gap definition surface and such that the inclination of the rotation body during rotation is restricted mainly by dynamic pressure generated in the thrust dynamic-pressure gap. This defines, for example, a dynamic-pressure bearing having a short axial length as shown in FIG. 3. When a bearing member serving as a rotation body inclines, the inclination is corrected by the action of dynamic pressure generated in the thrust dynamic-pressure gaps.
Dynamic-pressure grooves may be formed on the dynamic-pressure gap definition surface. For example, formation of known dynamic-pressure grooves on the outer circumferential surface, which serves as the radial dynamic-pressure gap definition surface, of a rotary spindle can realize far smoother rotation. As shown in FIG. 2(a), a plurality of dynamic-pressure grooves can be formed on the outer circumferential surface of the spindle (on the radial dynamic-pressure gap definition surface) while being arranged at predetermined intervals along the circumferential direction. In the embodiment of FIG. 2(a), linear grooves are arrayed while being inclined at a certain angle with respect to a generatrix of the outer circumferential surface of the spindle. However, dynamic-pressure grooves in any other known form can be used. For example, dynamic-pressure grooves in a so-called herringbone form can be used. Specifically, angle (boomerang-like) grooves are formed on the outer circumferential surface at predetermined intervals along the entire circumference such that the tips of the grooves are located on a circumferential reference line. Also, as shown in FIG. 2(b), dynamic-pressure grooves may be formed on the surface of a thrust plate (on the thrust dynamic-pressure gap definition surface). In FIG. 2(b), a plurality of curved grooves are formed on the surface of the thrust plate while being arranged at predetermined intervals in the circumferential direction of the thrust plate, which grooves are curved such that the distance between the center of the thrust plate and a point on each groove reduces gradually toward the inner end of the groove.
The dynamic-pressure bearing of the present invention can be effectively used with, for example, a spindle for rotating a hard disk of a hard disk drive, a spindle for rotating a disk of peripheral equipment, such as a CD-ROM drive, an MO drive, or a DVD drive, for computer use, and a spindle for rotating a polygon mirror of a polygon scanner for use in a laser printer, a copying machine, or a like machine. A bearing used in a rotational drive unit of such precision equipment is subjected to high-speed rotation at a speed of, for example, 8,000 rpm or higher (in some cases, even at a speed of 10,000-30,000 rpm or higher). Application of the present invention to such a bearing enables stable maintenance of generated fluid dynamic-pressure at high level to thereby effectively yield the effect of reducing vibration or the like. Also, the present invention provides a motor having a bearing in which the above-described ceramic dynamic-pressure bearing is used in a rotation output section. Further, the present invention provides a hard disk drive comprising the above-mentioned motor having a bearing and a hard disk to be rotationally driven by the motor as well as a polygon scanner comprising the above-mentioned motor having a bearing and a polygon mirror rotationally driven by the motor.
Alumina ceramic can be mixed with zirconia ceramic to obtain a composite ceramic material having high toughness. A product of such a composite ceramic material is formed in the following manner. A ceramic powder which contains either alumina or zirconia as a ceramic component of the highest content and the other as a ceramic component of the second highest content is formed into a green body, which is then fired to become a composite ceramic product. Preferably, zirconia ceramic is contained in an amount of 5-60% by volume based on the amount of alumina ceramic.
The composite ceramic material may contain alumina ceramic as matrix and an electrically conductive, inorganic compound phase whose metal cation component is at least any one of Ti, Zr, Nb, Ta, and W. A product of such a composite ceramic material is formed in the following manner. A material powder for forming matrix ceramic is mixed with a material powder for forming the electrically conductive, inorganic compound phase. The resulting mixed powder is formed into a green body, which is then fired to become a composite ceramic product. The electrically conductive, inorganic compound phase contained in a ceramic product imparts electrical conductivity to the ceramic product and thus enables the ceramic product to undergo electric discharge machining, such as wire-cut electric discharge machining. Imparting electrical conductivity yields an antistat effect.
The electrically conductive, inorganic compound can assume the form of at least any one of a metal nitride, a metal carbide, a metal boride, and a metal carbonitride which contain, as a metal cation component, at least any one of Ti, Zr, Nb, and Ta, as well as tungsten carbide. Specific examples of the electrically conductive, inorganic compound include titanium nitride, titanium carbide, titanium boride, tungsten carbide, zirconium nitride, titanium carbonitride, and niobium carbide. Preferably, the composite ceramic material contains the electrically conductive, inorganic compound phase in an amount of 20-60% by volume in order to attain sufficient enhancement of electrical conductivity while maintaining strength and fracture toughness. When the above-described composite ceramic is used, the previously described alumina content or sintering aid content is not of the composite ceramic, but of the alumina ceramic serving as a matrix.